Quenching of diphenylmethyl radical fluorescence by cyanoaromatics and phenols

Article abstract of DOI:10.1007/s11164-014-1914-0

We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical (²Ph₂CH^·) in acetonitrile at 25 °C. We

Full text of DOI:10.1007/s11164-014-1914-0

Res Chem Intermed
DOI 10.1007/s11164-014-1914-0
Quenching of diphenylmethyl radical fluorescence
by cyanoaromatics and phenols
Douglas Cyr Paritosh Das
•
Received: 3 October 2014 / Accepted: 22 December 2014
Springer Science+Business Media Dordrecht 2015
ꢀ
Abstract We have used double-laser nanosecond laser flash photolysis to study the
kinetics of bimolecular quenching of the lowest doublet excited state of the transient
2
ꢀ
diphenylmethyl radical ( Ph CH ) in acetonitrile at 25 ꢁC. We chose a series of
2
cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted
phenols as donor-type quenchers. The observed bimolecular quenching rate constants
6
k ) are in the range 2 9 10 –2 9 10
10
-1 -1
5
for cyanoaromatics and 1 9 10 –
(
M
for phenols. The Hammett plots of the k values for these two series
s
Q
9
9 10 M
-1 -1
3
s
Q
have positive and negative slopes, respectively, and establish the enhanced behavior of
the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cy-
2
anoaromatics, the trend in the dependence of Ph CH * quenching rate constants on
ꢀ
2
the free energy change of electron transfer (DG ) coincides well with that of Rehm–
el
Weller data on acceptor-type and donor-type quenchers of singlet excited states. An
approximate fit of the k data for cyanoaromatics to the Agmon–Levine model of
Q
-
electron transfer gave a value of 4.5 kcal mol for the free energy of activation at
1
DG = 0. For phenols, the trend in the dependence of k on the free energy change of
el
Q
electron transfer deviates significantly from that of cyanoaromatics data. This devi-
ation was traced to the use of irreversible oxidation potentials of phenols to calculate
DG . No isotope effect is observed from substitution of phenolic hydrogen by deu-
el
2
ꢀ
terium, i.e., k /kQ,D & 1. Phenolate ions quench Ph CH * with large rate constants,
–2 9 10
Q,H
0 -1 -1
2
1
1
M
s
, i.e., well in the limit of diffusion control.
Keywords Diphenylmethyl radical ꢀ Charge transfer ꢀ Doublet-doublet
fluorescence ꢀ Quenching
D. Cyr
Chevron Products Company, Richmond, CA 94801, USA
P. Das (&)
Department of Chemistry, Physics, and Engineering, Cameron University, Lawton, OK 73505, USA
e-mail: pdas@cameron.edu
123

D. Cyr, P. Das
Introduction
Since pioneering work by Weller [1], charge-transfer interactions of photo-excited
states of both organic and inorganic systems have been the subject of numerous
studies [2–26]. The high level of interest in such interactions is because they often
initiate chemical transformations which, if occurring from ground states, would be
energetically uphill. Not surprisingly, a substantial amount of research has been
conducted on utilization of photo-induced electron transfer to initiate chemistry by
involvement of a variety of transient intermediates, namely, singlet and triplet
excited states, radical ion pairs, exciplexes, solvated radical ions, and radicals [11,
16, 17, 19–24, 29, 30]. In addition, theoretical aspects [12, 14, 28, 31–34] of
electron transfer rates and the dynamics of such processes on ultrafast time scales
[
7, 15, 17, 21, 22, 25–27, 34] have recently attracted much attention.
In this paper, we present the results of a study, by dual-pulse, laser flash
photolysis, of quenching of the fluorescence of the transient diphenylmethyl radical
in fluid solutions by a series of cyanoaromatic compounds and by several phenols.
Whereas the former are well-recognized as acceptor-type quenchers, the latter are
meant to serve as electron and/or hydrogen-atom donors (established as effective
quenchers of aromatic carbonyl triplets) [35–37]. Studies of the reactivity of the
doublet excited state of transient radicals are a novelty compared with the vast
amount of work involving the singlet and triplet excited states [2–37].
During an early stage of this work, a paper was published by Scaiano et al. [38]
presenting the results of an exploratory study of the intermolecular reactivity of the
2
ꢀ
photo-excited diphenylmethyl radical ( DPM *). It reported rate constants for
reactions with a few selected quenchers (e.g., methyl viologen, triethylamine, and
1
,4-cyclohexadiene). In a follow-up paper, Scaiano et al. [39] reported the rate
2
ꢀ
constants for quenching of DPM * by a series of acceptor-type molecules and
showed that electron transfer from the excited radical to the latter usually occurs in a
facile manner. In this work, however, no attempt was made to correlate the rate
constant with the Gibbs energy change of electron transfer. Other than these two
studies, as far as we are aware, no systematic investigation of the charge-transfer
interactions of the doublet excited state of diphenylmethyl radical with a series of
structurally related quenchers has been reported. We note, however, that a related
investigation of the quenching of diphenylmethyl radical fluorescence by several
stable radicals [40] and that of excited states of a-CN-substituted benzyl, naphthyl,
and diphenylmethyl radicals [41] have implicated electron transfer in the quenching
mechanism.
Experimental
Materials
1
,1,3,3-Tetraphenylacetone (TPA) was prepared and purified by use of procedures
reported in the literature [38, 42]. Diphenylmethane (DPME) from Aldrich was used
as received. Synthesis and purification of 1,4-dicyanonaphthalene are described
1
23

Diphenylmethyl radical fluorescence quenching
elsewhere [43]. Tetracyanoethylene (Aldrich) was sublimed before use. The other
cyanoaromatic compounds, all from Aldrich, were either distilled under reduced
pressure or recrystallized from toluene (except 4-cyanobenzoic acid and 1,2,4,5-
tetracyanobenzene, which were recrystallized from ethanol). Acetonitrile (Aldrich,
Gold Label) was distilled over P O under an atmosphere of nitrogen. The sources
2
5
of phenols, and methods for their purification, are given elsewhere [3, 35, 44].
Nanosecond laser flash photolysis
The nanosecond laser flash photolysis experiments were performed at the Radiation
Laboratory, University of Notre Dame. A description of the kinetic spectropho-
tometer and the data collection system is available in previous publications from the
Radiation Laboratory [45–47]. The outputs from two laser systems were used: UV-
4
00 Molectron nitrogen (337.1 nm, 2–3 mJ, *8 ns) and Lambda-Physik EMG 101
excimer (Xe–Cl, 308 nm, B20 mJ, *20 ns). The time periods for observation of
transient species by fluorescence or absorbance were typically 100 ns–50 ls. For
most experiments in which the fluorescence of diphenylmethyl radical was
monitored, use was made of 1-cm 9 1-cm or 0.3-cm 9 1-cm quartz photolysis
cells in which two laser excitation pulses met at a right angle (with the monitoring
light blocked). For some experiments in which transient absorption measurements
were intended, a front-face configuration was used at an angle of *20ꢁ between the
directions of the laser pulses and the monitoring light. The time-resolved emission
or transient-absorption spectra were recorded on a wavelength-by-wavelength basis
in experiments in which the solution for photolysis was continuously fed from a
reservoir into a rectangular quartz photolysis cell (path length *3 mm). For
kinetics measurements at specific wavelengths, static cells (quartz, path length
usually 2 or 10 mm) containing 2–4 mL of the solutions were used. All
measurements were performed at 25 ꢁC in solutions deoxygenated by bubbling
with high-purity argon or nitrogen. The ground-state absorbances of the solutions at
laser excitation wavelengths were typically 0.2–0.4. The ground-state absorption
spectra were recorded in a Cary 219 spectrophotometer with a 1-nm bandpass.
In the dual-laser-pulse experiments, the excimer laser system was used to provide
the primary ‘‘synthesis’’ pulses at 308 nm to generate diphenylmethyl radical. This
was operated in tandem with the nitrogen laser system providing the ‘‘photolysis’’
pulses at 337.1 nm that photo-excited the radical. The 337.1-nm laser pulses were
delayed from the 308-nm pulses by 3–6 ls. The 308/337.1 nm combination of
synthesis and photolysis laser pulses have been found suitable in several of the
studies reported in the literature [38–40] on the photophysics and/or photochemistry
of diphenylmethyl radical.
Results
2
We used two methods to generate diphenylmethyl radical ( DPM or Ph CH ). The
ꢀ
2
ꢀ
2
first method made use of direct laser flash photolysis (308-nm) of 1,1,3,3-
123

D. Cyr, P. Das
2
tetraphenylacetone (TPA) causing it to decompose into Ph CH , which decayed
ꢀ
2
slowly over a ls time scale.
1;3
ꢁ
TPA þ hm ! TPA
ð1Þ
1
;3
ꢁ
2
ꢀ
TPA ! 2 Ph2CH þ CO
ð2Þ
2
This method is commonly used [38–41, 48] for facile photogeneration of DPM .
ꢀ
2
As shown in Fig. 1A, the transient diphenylmethyl radical ( DPM ) has a sharp,
ꢀ
-
1
-1
intense absorption band system with kmax at *330 nm (emax = 44,000 M cm
in a variety of solvents) [49]. In acetonitrile at 25 ꢁC, the 337.1 laser pulse
excitation of the radical at 2–5 ls after its formation results in its fluorescence in
the 500–600 nm spectral region, with a decay lifetime of 270 ns. The fluorescence
spectrum recorded in a wavelength-by-wavelength manner is presented in Fig. 1B.
2
ꢀ
No change in the spectral features is noticed during the decay of DPM *. The
observed structural features of the doublet–doublet fluorescence including its 0–0
band at 525 nm and its lifetime agree well with literature reports [38, 40, 50, 51].
2
ꢀ
In the second method of photogeneration of Ph CH , we photoexcited 1,2,4,5-
2
tetracyanobenzene (TCB) at a concentration of 0.03–0.05 mM in acetonitrile with
3
08-nm pulses in the presence of 30–40 mM diphenylmethane (DPME). As shown
by us in a recent study [52], the radical-cation of DPME produced as a result of
electron transfer from it to TCB singlet excited state underwent deprotonation to
ꢀ
6 -1
Ph CH with a first-order rate constant of 1.5 9 10 s
2
.
2
ꢀ
Fig. 1 (A) Absorption and (B) 337.1-nm-induced fluorescence spectrum of DPM produced by 308-nm
laser flash photolysis of TPA in acetonitrile and monitored *5 ls after the 308-nm laser pulse. The
absorption spectrum in terms of the molar extinction coefficient was obtained by multiplying the transient
-
1
-1
absorbance data with
e
max/DAmax
,
with
e
max = 44,000 M cm
(taken from Ref. [49]) and
DAmax = observed absorbance at maximum wavelength (330 nm). For the purpose of plotting on the
2
ꢀ
left scale, the calculated e values for DPM have been divided by 30. (C, D) Ground-state absorption
spectra of 4-nitrobenzonitrile and 4-bromophenol, respectively, in acetonitrile. (E) Ground-state
2
absorption spectrum of 4-bromophenolate ion in 9:1 (v/v) acetonitrile–H O containing 0.020 M NaOH.
For the purpose of plotting on the left scale, the e values for 4-bromophenolate have been divided by 2
1
23

Diphenylmethyl radical fluorescence quenching
1
ꢁ
TCB þ hm ! TCB
ð3Þ
ð4Þ
ð5Þ
1
ꢁ
2
ꢂꢀ
2
þꢀ
2
TCB þ Ph2CH2 ! TCB þ Ph2CH
2
þꢀ
2
ꢀ
þ
Ph CH2 ! Ph CH þ H
2
2
The 337.1-nm laser excitation of the radical several microseconds after the
08-nm ‘‘synthesis’’ pulse enabled observation of emission from it. A minor
3
disadvantage of this method was that the observed lifetime of radical fluorescence
was shorter (i.e., *200 ns in the presence of 0.05 mM TCB) owing to quenching of
the fluorescence by the cyanoaromatic acceptor (TCB). In addition, this method was
not useful in nonpolar solvents, e.g., benzene. All of our quenching studies with
2
ꢀ
phenols were performed with TPA as the photolysis substrate for DPM .
In Fig. 2 are presented four representative decay traces of fluorescence from
2
ꢀ
DPM in the presence and absence of two quenchers, 4-cyanobenzoic acid
acceptor-type) and 4-methoxyphenol (donor-type). Evidently, the presence of these
(
Fig. 2 Representative kinetic traces of diphenylmethyl radical fluorescence from 337.1-nm laser
excitation of the radical produced by 308-nm laser flash photolysis of: (A) 0.05 mM TCB in the presence
of 0.060 mM DPM, (B) 0.05 mM TCB in the presence of 0.060 mM DPM and 12.8 mM 4-cyanobenzoic
acid, (C) 0.78 mM TPA, and (D) 0.78 mM TPA and 7.80 mM 4-methoxyphenol. Solvent: acetonitrile at
25 ꢁC in all cases
123

D. Cyr, P. Das
quenchers at millimolar concentrations resulted in shortening of the observed
fluorescence lifetime. As the dashed line shows for each of the four traces, the
2
quality of fitting of the data to first-order decay kinetics was excellent (R C 0.99).
The ground-state absorption spectra of the quenchers studied do not reveal any
significant absorption at the photolysis laser wavelength (337.1 nm). This is evident
from Fig. 1, which shows the spectra of three quenchers, 4-nitrobenzonitrile,
4
-bromophenol, and 4-bromophenolate ion (Fig. 1C, D, and E, respectively). Of all
the quenchers studied, the absorption spectra of these three are among the most
extended to the red. The maximum concentrations used in quenching studies with
these and other, similar, very efficient quenchers (3,5-dinitrobenzonitrile, TCB, 1,4-
dicyanonaphthalene, phenolate, and 4-cyanophenolate ions) were so low (submil-
limolar levels) that absorption of the 337.1 nm photons by them was insignificant.
In any case, control experiments were performed in several such cases with the
3
without the DPM precursors (TPA or TCB/DPME) in solution. These showed that
37.1 nm laser pulses alone without the synthesis laser pulses (308 nm) and with or
ꢀ
2
2
ꢀ
interference with the DPM fluorescence from any emission from the quenchers
because of their excitation was either absent or negligible.
Except for phenolate ions, ground-state absorptions by the quenchers were also
negligible at the synthesis laser wavelength, 308 nm. Relative to phenols, the long-
wavelength absorption spectral band systems of the corresponding phenolate ions
are significantly red-shifted and have higher molar absorption coefficients (for
2
ꢀ
example, Fig. 1D, E). However, the DPM * quenching rate constants of phenolate
ions are very large (i.e., at the limit of diffusion control, vide infra) and the
maximum concentrations of these in quenching measurements are quite low
(
B0.2 mM). Under this condition, even with 4-bromophenolate ion, which has
3
-1
-1
e * 2910 M cm at 308 nm, the fraction of the laser light directly absorbed
by the phenolate ion is less than 30 % of that absorbed by TPA. The only
undesirable effect of this direct light absorption by the quencher is that the transient
2
ꢀ
absorbance of the DPM and the intensity of its 337.1-nm-induced fluorescence are
reduced. It should be noted that the photolysis by the relatively intense 308-nm laser
2
ꢀ
pulses results in some emission as a result of the DPM produced and excited by the
same laser pulse. However, this was not suitable for quenching studies owing to
interference from laser scatter and short-lived spurious emissions, some of which
probably originated from the shared small absorption of the light by the quenchers.
2
ꢀ
The decay traces of the emission of DPM in the presence of quenchers were
often monitored at multiple wavelengths covering the fluorescence spectrum of the
radical at 500–600 nm. No differences were noted in the measured lifetimes at
different wavelengths. This was indicative of the absence of interference from
emission, if any, from the quenchers and the lack of involvement of a radiative
2
ꢀ
ꢀ
DPM /quencher exciplex. The lack of evidence of exciplex formation between
DPM * and acceptor-type quenchers was also noted by Scaiano et al. [39].
2
Kinetics
We determined the bimolecular quenching rate constants (k ) by measuring the
Q
2
pseudo-first-order rate constants (kobs) of DPM fluorescence decay as a function of
ꢀ
1
23

Diphenylmethyl radical fluorescence quenching
quencher concentration. On the basis of Eqs. 7–9, below, the pseudo-first-order rate
constant (kobs) for decay of the fluorescence is expressed in terms of Eq. 10.
2
ꢀ
2
ꢀꢁ
DPM þ hm ! DPM
ð6Þ
ð7Þ
ð8Þ
k7
2
ꢀꢁ
2
ꢀ
DPM ꢂ! DPM
k8
2
ꢀꢁ
2
ꢀ
DPM ꢂ! DPM þ hmf
kQ
2
ꢀꢁ
DPM þ Q ꢂ! ProductðsÞ
ð9Þ
kobs ¼ k0 þ kQ½Qꢃ; where k0 ¼ k7 þ k8
ð10Þ
In Eq. 10, k is the sum of the nonradiative and radiative rate constants, k and k ,
8
0
7
2
respectively, of the decay of the radical. Thus, 1/k is the decay lifetime of DPM
ꢀ
0
fluorescence in the absence of a quencher. kobs is the observed pseudo-first-order
decay rate constant of the fluorescence in the presence of a quencher and k is the
Q
bimolecular rate constant for quenching associated with Eq. 9. On the basis of
Eq. 10, a plot of kobs against quencher concentration [Q] would be linear with a
2
slope equal to k . In experiments in which DPM was produced via photosensi-
ꢀ
Q
tization by TCB, the quenching component arising from the presence of TCB was
implicit in the measured k , that is, k = k ? k ? kQ,TCB[TCB], with [TCB] held
0
0
7
8
constant while the concentration of the intended quencher (Q) was varied.
Figures 3 and 4 show two representative sets of plots based on Eq. 10 for
cyanoaromatic and phenolic quenchers, respectively. Our results usually fitted
2
Eq. 10 reasonably well with R C 0.98. In some cases, for example with some of
2
the poor cyanoaromatic quenchers (e.g., benzonitrile), the R values were in the
range 0.96–0.98. The k data obtained from the slopes of the linear fits, for example
Q
those shown in Figs. 3 and 4, are presented in Tables 1 and 2. With two of the
Fig. 3 Representative plots based on Eq. 10, with cyanoaromatics as quenchers of diphenylmethyl
radical fluorescence in acetonitrile. To minimize clutter, quencher concentrations, [Q], have been
multiplied by factors, f, as shown
123

D. Cyr, P. Das
Fig. 4 Representative plots based on Eq. 10, with phenols as quenchers of diphenylmethyl radical
fluorescence in acetonitrile
phenolic quenchers, hydroquinone and 4-methoxyphenol, the quenching was also
studied in 9:1 (v/v) CH CN–H O and CH CN–D O. In CH CN–D O, the weakly acidic
3
2
3
2
3
2
phenols, withpK valuesinthe range 9–11[53], were expectedtoundergo deuterationas
a
shown in Eq. 11. This enabled observation of an isotope effect, if any, from deuterium.
ArOH þ D O ꢀ ArOD þ HOD
ð11Þ
2
In addition, for three of the phenols, i.e., those with methoxy, bromo, and cyano
substituents in the 4-position, the quenching rate constants were measured in
aqueous acetonitrile containing 0.02 M NaOH. The formation of the phenolate ions
in the strongly basic medium enabled us to compare quenching by phenols with that
by their anions.
ꢂ
ꢂ
ArOH þ OH ! ArO þ H2O
ð12Þ
2
ꢀ
The kinetic data for DPM * quenching by deuterated phenols and phenolate ions
are also included in Table 2. Notably, deuteration has almost no effect on the rate
constants. However, with rate constants in the limit of diffusion control, the phenolate
anions prove to be much stronger quenchers than the corresponding phenols.
In the ‘‘Discussion’’ section, below, we will establish the electron-transfer nature
of the quenching mechanism for both groups of quenchers. The scheme that is
widely applied [1, 3, 12, 14, 61, 62] for such a mechanism is shown below.
Scheme 1 Electron transfer quenching steps
1
23

Diphenylmethyl radical fluorescence quenching
Table 1 Bimolecular rate constants for quenching of diphenylmethyl radical fluorescence by cyano-
aromatics in acetonitrile at 25 ꢁC
c
Quencher
-E1/2
,
-E1/2
,
DGel
(kcal mol
Highest quencher
concentration
used (mM)
k
Q
(10 M
a
b
-1
9
-1 -1 d
s )
red (V)
red (V)
)
Tetracyanoethylene
0.17
0.96
0.24
-0.59
-0.66
-0.88
-1.33
-1.54
-1.64
-1.66
-1.75
-1.80
-2.03
-2.32
-2.35
-2.38
-2.58
-53.2
-34.1
-32.5
-27.4
-17.0
-12.2
-9.86
-9.46
-7.38
-6.23
-0.927
5.76
0.14
0.11
0.50
0.51
0.60
1.0
24.0
24.2
3
1
4
1
4
1
4
1
1
4
4
,5-Dinitrobenzonitrile
,2,4,5-Tetracyanobenzene 1.02
26.4
-Nitrobenzonitrile
,4-Dicyanonaphthalene
-Cyanobenzoic acid
,4-Dicyanobenzene
-Cyanopyridine
1.25
26.7
19.9
1.91
1.97
2.03
2.12
2.12
2.4
11.1
0.93
0.56
0.90
0.94
8.8
12.4
7.43
,2-Dicyanobenzene
,3-Dicyanobenzene
-Chlorobenzonitrile
-Fluorobenzonitrile
12.5
8.96
0.19
2.69
2.74
2.75
2.95
100.
555
0.0097
0.0021
0.0021
\0.0002
Benzonitrile
6.51
4
4
a
-Toluonitrile
-Anisonitrile
7.14
474
11.8
1,000
4
Relative to Ag/AgClO (0.1 M) in N,N-dimethylformamide. Taken from Ref. [54]
b
Relative to SCE in acetonitrile. The data for tetracyanoethylene, 1,4-dicyanobenzene, and benzonitrile
were taken from Ref. [1]. The data for 1,2,4,5-tetracyanobenzene and 1,4-dicyanonaphthalene were taken
from Refs. [55] and [56], respectively. The data for the other quenchers were calculated by adding 0.37 to
4
the corresponding data in N,N-dimethylformamide relative to Ag/AgClO (0.1 M) in the second column.
This ‘‘correction’’ factor, i.e., 0.37 ± 0.04 was estimated from the average of the differences between E1/2
values for the cyanoaromatic systems for which data were available under both conditions
c
2
ꢀ
DGel values were calculated by use of Eq. 13a. For the oxidation potential of DPM , the value of
.35 V (vs. SCE in acetonitrile) [57, 58] was used
0
d
±
15 %
In Scheme 1, the key property is the Gibbs energy change (DG ) of electron
el
2
ꢀ
transfer at the reaction distance between DPM * and Q (i.e., on going from A to B).
As in Ref. [1], the DGel values for the quencher systems that we studied were
calculated by use of Eqs. 13a and 13b.
Acceptor type Q:
DGelꢀ ¼ ꢂDEꢁ þ 23:06ðEox;Rꢀ ꢂ Ered;QÞ þ WR þ WP
ð13aÞ
0
;0
Donor type Q:
DGelꢀ ¼ ꢂDEꢁ þ 23:06ðEox;Q ꢂ Ered;Rꢀ Þ þ WR þ WP
ð13bÞ
0
;0
In Eqs. 13a and 13b, DE0,0* is the excitation energy of the DPM radical, Eox and
Ered are the oxidation and reduction potentials (V), respectively, and the W values
are the coulomb work in bringing together reactants and products to form the
association complex (A) and ion-pair (B), respectively. From the location of the
123

D. Cyr, P. Das
1
23

Diphenylmethyl radical fluorescence quenching
Q
Fig. 5 Plot of log(k ) as a function of free energy change (DGel) of electron transfer at the reaction
distance, using data from Ref. [1] and results from this work
(
0,0) band of DPM fluorescence at 525 nm, DE * is calculated to be
0
,0
-
1
2
5
4.5 kcal mol . W for the ion-pair (B) in question is given by -e /(eR), where
P
e is the dielectric constant of the medium and R is the distance between the ions in
˚
the ion-pair, a reasonable estimate of which is 7 A [12]. For uncharged reactant
pairs under examination, W = 0. For acetonitrile and 9:1 (v/v) acetonitrile–H O
R
2
-
W is calculated to be -1.3 and -1.1 kcal mol , respectively. By using Eqs. 13a
1
P
and 13b and redox potential data available in the literature [54–60], we calculated
the Gibbs energy changes for electron transfer (DG ). The DG data are also
el
el
presented in Tables 1 and 2. Note that with phenolate ion quenchers, W = W = 0.
R
P
In Fig. 5, we have plotted log(k ) as a function of DG . For purposes of
Q
el
comparison, we have included in this figure the large set of data published by Rehm
and Weller [1] for 67 quencher–singlet-excited-state pairs, in acetonitrile, for which
electron transfer was recognized as the mechanism of quenching. Clearly, on the
plot in Fig. 5, the trend and the location of our kQ data for cyanoaromatic and
phenolate-ion quenchers overlap the Rehm–Weller data very well [1]. However, the
kQ data for phenols exhibit a distinct departure.
Phenols are known to be electron-rich donors that act as efficient quenchers of
photo-excited states [3, 35, 63]. One would expect their quenching rate constants to
depend on the electron-withdrawing and electron-releasing nature of their substit-
uents. In Fig. 6A, B, we have plotted log(k ) as a function of the Hammett
Q
?
constants, r and r , of para substituents. Although the linearity of the plots is not
?
good, the negative slopes from the best fits, with q = -4.0 and q = -2.4, imply
that the phenols serve as electron donors and establish the electrophilic nature of
2
ꢀ
DPM *. A similar inference can also be drawn on the basis of the dependence of k
values on oxidation potentials. For comparison, for quenching of carbonyl triplets
Q
by phenols in 1:1 (v/v) acetonitrile–water, the Hammett plot has been reported to
?
give a value of -1.4 for q [63].
As with phenols, in Fig. 7A, B, we have plotted log(k ) for monosubstituted
Q
?
benzonitriles, X–C H –CN, as a function of the Hammett constants (r and r ) of
6
4
the substituent X. The linearity of the plots is better than for the phenols. As
123

D. Cyr, P. Das
Q
Fig. 6 Hammett plots for quenching of diphenylmethyl radical fluorescence by phenols. (A) log(k ) as a
?
?
Q
function of r; (B) log(k ) as a function of r . The r and r -data were taken from Ref. [64]
expected for acceptor-type quenchers, from the slopes of the plots we obtained
?
positive values for q and q , namely, ?5.1 and ?4.3, respectively.
Net electron transfer yields
To determine whether the electron transfer involving either set of the quenchers
manifested itself in the formation of solvated radical or radical–ion products, we
conducted a few laser flash photolysis experiments in the absorption mode. In
experiments with TCB as the quencher at 0.2 mM in acetonitrile, we monitored
transient absorbance changes from the action of the photolysis pulse (337.1 nm) at
3
maxima of Ph CH , Ph CH , and TCB with molar extinction coefficients of
25–330, 440, and 460 nm. These wavelengths correspond to the absorption
2
ꢀ
?
2
-ꢀ
2
2
4
.4 9 10 [49], 4.4 9 10 [49], and 1.4 9 10 M cm [65], respectively. We
4
4
-1
-1
4
did not observe any non-negligible bleaching of absorbance at 325–330 nm or
formation of absorbance at 440 and 460 nm that could be attributed to the electron-
?
transfer-derived transient products, Ph CH and TCB , respectively. Similar
2
-ꢀ
2
negative results were also obtained with 1.5 mM 1,4-dicyanobenzene as the
quencher in acetonitrile. We concluded that the yield (g ) of solvated ion or radical-
ion from full electron transfer was small, if not negligible, with TCB and
el
1
23

Diphenylmethyl radical fluorescence quenching
Fig. 7 Hammett plots for quenching of diphenylmethyl radical fluorescence by monosubstituted
?
?
Q Q
benzonitriles. a log(k ) as a function of r; b log(k ) as a function of r . The r- and r -data were taken
from Ref. [64]
1
quencher for Ph CH * in 4:1 (v/v) acetonitrile–water, Scaiano et al. [38]. reported
,4-dicyanobenzene as the quencher. It should be noted that with methyl viologen as
2
ꢀ
2
estimates of the yield (g ) of full electron transfer to methyl viologen at 70–80 %.
el
Also, with several acceptors, for example CCl , the radical anions of which undergo
reductive dehalogenation, Scaiano et al. [39] measured gel values at 20–60 % by
4
?
monitoring the electron-transfer-derived diphenylmethyl carbocation (Ph CH ) in
2
1
acetonitrile. Specifically, with 1,4-dicyanobenzene as the quencher, the reported
1
2?
The reduction potential of methyl viologen (MV ) is -0.21 V (in aqueous solution, vs. SCE) [66]. On
ꢀ
ꢁ
2?
the basis of this value, DGel for electron transfer between Ph2CH and MV is calculated to be
-
electron-transfer-derived product (MV ) with MV as the quencher in aqueous acetonitrile is
-
1
-1
43 kcal mol , which is comparable with DGel for TCB (-33 kcal mol ). The large yield of the
ꢀ
? 2?
?
ꢀ?
explainable in terms of the fact that both ions in the product ion-pair, (Ph
2
CH ꢀꢀꢀMV ), are positively
CH is very short-lived in aqueous acetonitrile,
the bimolecular rate constant for the reactivity of the cation with water is reported to be
?
charged, rendering ion separation facile. In addition, Ph
2
8
-1 -1
?
1
.3 9 10 M
s
i.e., separation) of MV . In contrast with use of methyl viologen, the product ion-pairs in the course of
2
(in acetonitrile) [67]. The rapid decay of Ph CH would also facilitate the formation
ꢀ
?
(
2
ꢀꢁ
the electron-transfer quenching of Ph2CH by cyanoaromatics consist of oppositely charged ions. The
relatively high yields of electron transfer with quenchers the radical anions of which undergo rapid
dehalogenation are also explained by the facile separation of diphenylmethyl carbocation from neutral
quencher-derived radicals.
123

D. Cyr, P. Das
[
39] small value of 3 % for gel is not in serious contradiction with our lack of
observation of net electron transfer with this quencher (because of large errors in gel
measurements).
We selected 4-methoxyphenol as a representative quencher to investigate the
extent of full electron transfer, if any, for phenols. The expected product of electron
transfer as a result of deprotonation of the phenol radical-cation (4–CH O–
3
?
ꢀ
C H OH ) is the 4-methoxyphenoxy radical, which absorbs strongly at 405–410 nm
6
4
(
in acetonitrile) [35, 63] with emax approximately 1.4 times larger than that of the
3 3 -1 -1
benzophenone ketyl radical at *545 nm (emax = 3 9 10 –4 9 10 M cm in a
variety of solvents) [68, 69]. The other product from electron transfer to DPM *
-
would be the diphenylmethyl carbanion, Ph CH , known to have absorption maxima
2
ꢀ
2
at 400–450 nm (as ion-pairs in a variety of amine and ether solvents) [70, 71]. Our
two-laser experiments with 10 mM 4-methoxyphenol in acetonitrile resulted in
ꢀ
enhancement of absorption at 410–415 nm as a result of photolysis of Ph CH by
2
337.1-nm laser pulses in the presence of the phenol. A comparison of this absorbance
change with that resulting from end-of-pulse bleaching at 330 nm (induced by the
3
extinction coefficients of Ph CH and 4-methoxyphenoxy radical, gave an approx-
37.1-nm photolysis pulse in the absence of the quencher), coupled with use of molar
ꢀ
2
2
imate estimate of 0.4 ± 0.1 for the yield of net electron transfer (gphenoxy). A similar
experiment with 4-methoxyphenol in benzene made use of the end-of-pulse transient
ꢀ
absorption due to Ph CH * at 405 nm (in the absence of the quencher) for
2
comparison with the transient absorbance due to the phenoxy radical at 415 nm. On
ꢀ
4
= 2.8 9 10 M cm [51], we estimated a higher value of
-1
-1
the basis of e
Ph2CH*
0
absorbance that could be attributed to the carbanion, Ph CH . This is not surprising,
.5 ± 0.1 for gphenoxy in benzene. In either solvent, we could not detect any transient
-
2
because of the negative pK (i.e., -4.7 in acetonitrile) [59] of the radical-cation of
a
4
diphenylmethane, Ph CH . Charge-transfer-induced hydrogen transfer from phenols
-methoxyphenol and the very large positive pK (32.2 in dimethyl sulfoxide) [72] of
a
2
2
2
to DPM * will be presented in more detail in the ‘‘Discussion’’ section, below.
ꢀ
Discussion
On the basis of the lowest singlet and triplet energies of both series of quenchers
2
under study, energy transfer from Ph CH *, with its lowest doublet energy at
ꢀ
2
-
1
5
4.5 kcal mol , should be energetically unfavorable. Among the quenchers studied,
only the two nitro-substituted aromatics in Table 1 have their lowest-energy
absorption band system extended beyond near-UV region into the visible. As shown
2
The phenoxy radical yields (gphenoxy) were estimated by use of the equation:
DAphenoxyePh CHꢀ
2
kobs
gphenoxy ¼ DA
0
bleach;Ph2CHꢀ
ephenoxy kobsꢂk0
where DAphenoxy is the absorbance change due to the 4-methoxyphenoxy radical at 415 nm following the
ꢀ
0
2
is the end-of-pulse absorbance change due to bleaching of Ph
ꢀ
decay of Ph
2
–CH *, DA
ꢁ
2
CH at
4
bleach;Ph2CH
3
7
30 nm, ephenoxy and ePh CHꢀ are the corresponding molar extinction coefficients (namely, 4.4 9 10 and
0 2
2
3
.0 9 10 M cm , respectively) [50, 51, 63], and kobs and k are the decay rate constants of Ph CH *
-1
-1
2
ꢀ
under quenching by the phenol and in its absence, respectively.
1
23

Diphenylmethyl radical fluorescence quenching
for 4-nitrobenzonitrile in Fig. 1C, this band system (i.e., the tail at 320–400 nm) is
-1
-1
very weak, with molar absorption coefficients in the range 2–50 M cm and,
presumably, owes its origin to an n, p* transition involving the nitro group. Clearly, it
does not overlap with the fluorescence spectrum of DPM (Fig. 1) Thus, energy
2
ꢀ
3
transfer to the singlet state of the quencher by a F o¨ rster mechanism does not occur.
The same is true for all of the quenchers studied, including the phenolate ions, the
absorption spectra of which are substantially red-shifted relative to phenols.
The triplet energies of benzonitrile [74], TCB [75], and phenol [76] are 77, 65,
-
1
and 82 kcal mol , respectively. Except for the nitro group, the substituents on
benzonitrile and phenol are unlikely to lower their energies sufficiently to make
2
doublet–triplet energy transfer from Ph CH * to them exoergic. The triplet energies
ꢀ
2
-
1
of nitrobenzene derivatives are reported [77] to be in the range 60–77 kcal mol
Thus, even for nitro-substituted benzonitriles, a quenching route through excitation
.
2
transfer from Ph CH * seems unimportant. Because doublet–singlet and doublet–
ꢀ
2
triplet energy transfer is an unlikely contributor to the quenching, the discussion
below will focus on electron transfer as the primary mechanism for quenching of
2
ꢀ
Ph CH * by cyanoaromatics and phenols.
2
Cyanoaromatics
The data set of quenching rate constants (Table 1) is relatively extensive for
cyanoaromatics and the plot of log(k ) as a function of DG coincides well with that
Q
el
of Rehm–Weller data (Fig. 5). Because of this, it seemed worthwhile to treat our
data on cyanoaromatics in terms of different electron-transfer models as applied to
similar quenching data in the literature [1, 3, 12, 14, 61, 62]. In a manner very
similar to that used by Hug and Marciniak [14], we attempted to fit the
4
cyanoaromatics quenching data into the expression for kQ derived on the basis
z
three conventional models, namely, Rehm–Weller, Agmon–Levine [78], and
of Scheme 1 and relating the barrier for electron transfer (DG ) to DG by use of
el
el
5
6
3
The standard measure of F o¨ rster interaction between energy donors and acceptors is in terms of a
radius, R , given by [73]:
0
2
Rfð mꢀ Þeð mꢀ Þ
6
0
9000 j5 ln 10
R ¼ Uf
4
4
dmꢀ
128s n N
mꢀ
2
ꢀ
where U
f
is the fluorescence yield of DPM * (= 0.29) [51], j is a dipole orientation factor, n is the
2
ꢀ
refractive index of the medium (1.334 for acetonitrile), fð mꢀ Þ is the emission spectrum of DPM *
(
normalized to unit area) and eð mꢀ Þ is the absorption spectrum of the quencher. In the absence of any
2 ꢀ
overlap between the fluorescence spectrum of DPM * and the absorption spectra of the quenchers under
study, R is always zero.
Application of steady-state approximations for the intermediates in Scheme 1 leads to the following
0
4
expressions for k
h
Q
and gel
.
ꢀ
ꢁiꢂ1
kꢂd
kꢂel
kQ ¼ kd 1 þ
1 þ
ksepþkbet
kel
ksep
gel ¼ ksep
þkbet
h
i1=2
2
5
6
z
DGel
4
z
2
DGel
2
Rehm–Weller [1]: DG
¼
z
þ DG ð0Þ
þ
:
el
el
"
!#
z
DGel ð0Þ
ꢂDGelln2
Agmon–Levine [78]: DG ¼ DGel þ
ln 1 þ exp
:
el
ln2
z
DG ð0Þ
el
123

D. Cyr, P. Das
Fig. 8 Plots of observed and computed log(k
transfer at the reaction distance, using the Agmon–Levine model
Q
) as functions of free energy change (DGel) of electron
7
Marcus [79] . These attempts were mostly less than satisfactory, mainly owing to
the lack of sufficient data at small |DG |. Figure 8 shows the fit using Agmon–
el
-
Levine model that led to an estimate of 4.5 kcal mol for the free energy of
1
activation at DG = 0. This value is substantially higher than that obtained by Hug
el
-
1
and Marciniak [14] by using Rehm–Weller data (3.3 kcal mol ).
Our preliminary attempts to fit our kinetic data for cyanoaromatics to diffusion-
based models in a manner similar to that in Ref. [14] were not rewarding. The same
was true for our attempt to fit the data to the Sandros–Boltzmann model [80]
recently applied by Farid et al. [27]. We plan to collect quenching and redox
potential data for other acceptor-type systems to re-examine the application of
recent theoretical models.
Phenols
As already noted, the phenols in our study constitute a series of donor-type
quenchers. In addition to the negative slope of the Hammett plots (Fig. 6),
involvement of charge transfer is indicated because, with 4-methoxyphenol as the
quencher, the electron-transfer-derived product, the 4-methoxyphenoxy radical, is
actually observed to form in high yields. Also, the corresponding quenching rate
constant decreases on going from acetonitrile to a nonpolar solvent (benzene) and
increases on going to the more polar solvent mixture, 1:9 (v/v) H O–CH CN.
2
3
Enhancement of the rate of quenching in aqueous acetonitrile is also observed with
-hydroxyphenol. Interestingly, however, for phenol, an opposite effect, i.e., a
decrease in k by a factor of two, is observed in the presence of 10 % (v/v) water in
4
Q
acetonitrile. A tentative explanation of this is that in aqueous acetonitrile both
2
phenol and Ph CH * are strongly hydrogen-bonded with water and this hampers the
ꢀ
2
2
quenching interaction. Hydrogen bonding of Ph CH * with water has, in fact, been
ꢀ
2
suggested by Scaiano et al. [38] as a possible reason for the lengthening of its
lifetime in aqueous acetonitrile. Also, a previous study [44] has shown that the rate
7
Marcus [79]: DGz
k
DGel
k
2
z
¼ 4
½1 þ
ꢃ , with k = 4DG ð0Þ:
el
el
1
23

Diphenylmethyl radical fluorescence quenching
constant for 4-methoxyacetphenone triplet quenching by several phenols is
substantially reduced when the latter are hydrogen-bonded to a variety of substrates.
Another example of the negative effect of H-bonding on reactivity is that observed
on the electrophilic reaction of diarylmethyl and triarylmethyl cations with primary
amines in aqueous acetonitrile [81]. The negative effect from water in this example
…
has been attributed to formation of a hydrated amine, RNH HOH, which is,
2
apparently unreactive (in contrast with free RNH ). Also, notably, the quenching
2
rate constants of carbonyl triplets by phenols are substantially smaller in wet
acetonitrile than in benzene [35].
2
One possible mode of the quenching of Ph CH * by phenols would be via direct
ꢀ
2
2
ꢀ
hydrogen abstraction by DPM *. However, we rule out such a mode of quenching
because we do not observe any deuterium-vs.-hydrogen isotope effect, kQ,H/kQ,D
being essentially unity. In comparison, the k /k ratio has been found to be in the
H
D
range 3.6–4.8 for hydrogen abstraction from phenols by the tert-butoxy radical (in
tert-butyl alcohol) [82] and 1.2–4.5 for aromatic carbonyl triplet quenching by
phenols [35].
The spectacular enhancement of quenching rate constants on changing from
phenols to phenolate ions (Table 2) is not surprising. The oxidation potentials of
phenolate ions are substantially lower than those of the corresponding phenols [60].
2
ꢀ
This renders the DG values for Ph CH * quenching by phenolate ions extremely
el
2
negative and, therefore, highly favorable for quenching. Even for 4-cyanophenol,
2
which is essentially a nonquencher for Ph CH *, the rate constant for quenching by
ꢀ
2
-
the corresponding anion (4–CN–C H O ) is in the limit of diffusion control
6
4
(
Table 2). A similar increase in the quenching rate constants with phenolate ions
relative to those with phenols has previously been noted for carbonyl triplets [63]
and the tris(2,2-bipyridine)ruthenium(II) excited state [3].
Perhaps the most interesting observation regarding phenol quenching in this
study is that below the limit of diffusion control the dependence of k on DG does
Q
el
not follow the trend shown by the cyanoaromatics (Fig. 5). For the same DGel
values, the observed kQ values for quenching by phenols are substantially higher
than those for quenching by the cyanoaromatics. If one considers that the electron
2
ꢀ
transfer between Ph CH * and a phenol is associated with concomitant proton
2
8
transfer in the resulting ion pair (Eq. 14), the Gibbs energy change for the net
H-transfer would be given by Eq. 15.
2
ꢂ
þꢀ
2
ꢀ
ðPh2CH ꢀ ꢀ ꢀ PhOH Þ ! ðPh2CH2 ꢀ ꢀ ꢀ PhO Þ
ð14Þ
ð15Þ
e2
DGht ¼ DGen þ
ꢂ 2:303RTðpKa;Ph CH ꢂ pK
þꢀ
a;PhOH
Þ
2
2
eR
With phenol as a specific example, using pK values [59, 72] of -8.1 and 32.2 for
a
?
ꢀ
PhOH and diphenylmethane (Ph CH ), respectively, we calculate DG to be
2
2
ht
-
1
-43 kcal mol . Such a highly negative DG cannot justify the observed k values
for phenol well below the limit of diffusion control. In addition, with a value of -13
Q
for the pK of the radical cation of 4-cyanophenol [59], DG for this phenol is
a
ht
8
Such a mechanism for quenching via H-transfer is discussed in Ref. [1].
123

D. Cyr, P. Das
-
estimated to be nearly equally negative (-39 kcal mol ). However, 4-cyanophenol
1
2
ꢀ
is almost a nonquencher for DPM *. Thus, we are led to conclude that proton
transfer occurs as a very rapid step subsequent to electron transfer between the
2
phenol and DPM * and should be inconsequential to the observed kinetics.
ꢀ
One way to explain or reconcile the deviation of the k data for phenols from the
Q
plot in Fig. 5 would be to recognize that we have used irreversible oxidation
potential data of phenols to calculate DGel values. As shown in Ref. [60] for
oxidation potential of phenols in water, the reversible oxidation potentials of
phenols (i.e., the reversible reduction potentials of phenol radical-cations:
?
ꢀ
-
PhOH ? e ꢁ PhOH) are more positive that those of the phenolate ions by
0.7 V. If we use the corresponding value of 1.36 V (vs. SCE) for E instead of
*
1
/2,ox
-
1
1.74 V (Table 2), the calculated DG would be more negative by *9 kcal mol .
Such adjustment of the oxidation potential data for phenol and its substituted
el
analogs would move the plot corresponding to them in Fig. 5 to the left and make it
overlap with the plot for k values of cyanoaromatics and Rehm–Weller systems.
Interestingly, on the basis of consideration of highest occupied and singly
Q
2
occupied molecular orbitals involved in the photoexcitation of DPM , the electron
ꢀ
2
affinity of DPM * has been deemed to be unchanged relative to the ground state of
ꢀ
2
ꢀ
2
DPM [39]. Thus, DPM * is unlikely to be a better acceptor than DPM . However,
ꢀ
2
ꢀ
our quenching data with phenols do not support this conclusion.
Summary
By use of double-laser nanosecond laser flash photolysis we have performed a
kinetic study on bimolecular interactions of the lowest doublet excited state of the
transient diphenylmethyl radical in acetonitrile with a series of cyanoaromatic
compounds as acceptor-type quenchers and a series of phenols as donor-type
quenchers. The diphenylmethyl radical was produced by 308-nm laser photolysis of
either 1,1,3,3-tetraphenylacetone alone or 1,2,4,5-tetracyanobenzene in the presence
of diphenylmethane (DPME). In the latter case, the diphenylmethyl radical resulted
from deprotonation of the electron-transfer-derived radical cation of DPME. The
important findings from this work are:
•
The Hammett plots of quenching rate constants (k ) have negative slopes for
Q
phenols and positive slopes for cyanoaromatics, as expected for their interac-
ꢀ
2
tions with Ph CH * as donors and acceptors, respectively. These results
2
establish the enhanced charge-transfer behavior of the radical in its lowest
excited state as both an acceptor and a donor.
2
For cyanoaromatics, the trend in the dependence of Ph CH * quenching rate
ꢀ
•
2
constants on the free energy change of electron transfer (DG ) coincides well
el
with that of the data by Rehm and Weller [1] for a large set of acceptor-type and
donor-type quenchers of singlet excited states. Attempts made to fit the k data
Q
on cyanoaromatics with Rehm–Weller, Agmon–Levine, and Marcus models of
electron transfer led to less than satisfactory results. With specific restrictions
1
23

Diphenylmethyl radical fluorescence quenching
imposed, the best fit of the data to the Agmon–Levine model gave a value of
-
1
z
For phenols, no isotope effect was observed from substitution of phenolic
4
.5 kcal mol for the free energy change of activation, DG ð0Þ at DG = 0.
el
el
•
•
hydrogen by deuterium, that is, kQ,H/kQ,D & 1. This suggests that quenching by
2
direct hydrogen abstraction from phenols by Ph CH * is not significant.
ꢀ
2
For phenols, the trend in the dependence of kQ on the free energy change of
electron transfer (DG ) deviates substantially from that of the data by Rehm and
el
Weller [1] if irreversible oxidation potentials of phenols in acetonitrile are used
to calculate DG . However, the deviation essentially disappears if the oxidation
el
potentials are adjusted to reversible values.
As the data for 4-methoxyphenol suggest, the yields of net electron transfer in
•
•
2
the course of Ph CH * quenching by phenols are, or can be, modestly high. In
ꢀ
2
contrast, with cyanoaromatics, the yields of solvated ions from net electron
transfer seem to be small or negligible (within the errors of measurements), as
suggested by experiments with 1,4-dicyanobenzene and 1,2,4,5-tetracyanoben-
zene in acetonitrile.
On changing from phenols to phenolate ions, the kQ values increase spectac-
ularly and reach the limit of diffusion control. This increase occurs because the
exoergicity of electron transfer, as expressed by DG , is substantially higher for
el
the anions.
Acknowledgments Part of the work presented herein benefitted from generous financial support from
the Harvard L. and Judith D. Tomlinson of Duncan Endowed Lectureship in Physical Sciences at
Cameron University. We gratefully acknowledge the assistance of Dr A. Samanta with some of the work
in its early stage. Nearly all of the experimental work described in this report utilized laser flash
photolysis systems and other instrumental facilities at the Notre Dame Radiation Laboratory (NDRL).
P.D. is grateful to Drs Gordon Hug, Ian Carmichael (Director, NDRL), and G.N.R. Tripathi for help and
hospitality during part of the work.
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